An obstacle detection apparatus that detects an obstacle by transmitting and receiving an ultrasonic wave includes an ultrasonic sensor. The obstacle detection apparatus measures the time interval from a timing in which the ultrasonic sensor transmits an ultrasonic wave to a timing in which a reflected wave of this ultrasonic wave reflected by an object is received, to determine the distance to the object. Whether the object is an obstacle is determined based on the calculated distance to the object.
When there is an ultrasonic sensor of another device around the obstacle detection apparatus, crosstalk may occur. Crosstalk means reception, by the ultrasonic sensor of one's device, of a wave transmitted by the ultrasonic sensor of another device.
It is difficult to distinguish the wave transmitted from the other device from a reflected wave of the transmitted wave from the ultrasonic sensor of one's device. Therefore, crosstalk causes an error in the calculated distance, as a result of which the accuracy of obstacle determination is lowered. Such lowered accuracy may erroneously activate a driver assistance function that is executed based on the result of obstacle determination, such as setting off an alarm unnecessarily to draw attention of the driver to an obstacle.
In Patent Literature 1, to prevent crosstalk, an infrared sensor that transmits and receives an infrared beam is further provided, so as to synchronize the timing of transmitting an ultrasonic wave with that of other ultrasonic sensors.
However, providing an infrared sensor for preventing crosstalk causes a cost increase. To prevent crosstalk, the interval of transmitting a wave could be varied randomly. The time interval from a timing in which a wave is transmitted to a timing in which a reflected wave is received is not affected by the variation in the transmission interval. On the other hand, varying the time of transmitting a wave from the ultrasonic sensor of one's device randomly will cause the time lag in receiving a transmitted wave from the ultrasonic sensor of the other device to vary randomly. This is because the timing at which the ultrasonic sensor of the other device transmits a wave is not affected by the interval at which the ultrasonic sensor of one's device transmits a wave. Therefore, by varying the interval of transmitting a wave randomly, crosstalk can be distinguished.
For the crosstalk between obstacle detection apparatuses of the same type, the transmission interval of the ultrasonic sensor in the other obstacle detection apparatus would also be varied randomly. Even though the ultrasonic sensor of one's device varies its transmission interval, if the ultrasonic sensor of the other device also changes the transmission interval and this changed interval happens to be the same as that of the one's device, the crosstalk cannot be distinguished.
In order to minimize the possibility that these transmission intervals match, there needs to be as many transmission intervals as possible. With a large number of transmission intervals, however, the longest transmission interval will become longer, as compared to a case in which there are fewer transmission intervals. This is because the longest transmission interval cannot be made shorter than a length determined by the device's minimum processing cycle multiplied by the number of transmission intervals provided. Selecting one from this large number of transmission intervals randomly controls the transmission/reception cycle to be long.